Not really hard, but you do want to be thorough. There are products called "Deglossers" and solvent washes specifically to remove wax from fiberglass (The gelcoat/paint is often the first part of a boat that goes in a mold, on top of a layer of mold release wax. This wax has to be removed before adding trim or touch-up paints). Acetone will work, but you have to use lots of rags or you'll just spread the wax around and it will re-harden once the acetone evaporates off.